SM420 transmission 1:1 final with 3:38.1 ring pinion 27" dia. Tires 1954 3100 PU. At 35mph on speedometer actual speed 40 mph about 5/6 mph difference. We need a ratio adapter for the transmission any suggestions or model numbers. Don't want to fool around with the speedometer itself a ratio adapter would be an easy fix and easy to remove.
Thanks!